Optoelectronic material and device application, and method for manufacturing optoelectronic material

ABSTRACT

An optoelectronic material, device applications, and methods for manufacturing the optoelectronic material are provided to make it possible to obtain stable characteristics without deterioration of luminescence over time in the atmosphere. The optoelectronic material is composed of a porous silicon the surface of which is nitrided to form a silicon nitride layer thereon. This allows a stable electroluminescence to be obtained, without oxidation of the surface of the porous silicon.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optoelectronic material anddevice application, and a method for manufacturing an optoelectronicmaterial, and more specifically, to an optoelectronic material formedfrom luminescent silicon (Si) —a substance with an inexhaustible supplyand that causes no environmental pollution—as its core, and furthercharacterized by excellent compatibility with Si-LSI technology,self-luminescence, and stability, and a manufacturing method therefore.

[0003] 2. Description of the Prior Art

[0004] As Si is an indirect transition semiconductor, and its bandgap isnear the infrared region, at 1.1 eV, it has not been thought possible touse it as a light emitting device in the visible region. In 1990,however, visible light emission at room temperature was confirmed withporous Si (e.g. L. T. Canham, Applied Physics Letters; Vol. 57, No. 10,1046 (1990)). Since that time, research on visible light emission atroom temperature with Si as the base material has become quite popular.As the vast majority of these reports have concerned porous Si, thisluminescent porous Si will be described as an example of the prior art

[0005] Basically, luminescent porous Si is formed by anodizing thesurface of a single-crystal Si substrate with a solution comprisinghydrofluoride mainly. Up to the present, photoluminescence (PL) has beenconfirmed at a number of wavelengths in the visible light region, from800 nm (red) to 425 nm (blue) . There have also been recent attempts togenerate electroluminescence (EL) through current injection excitation.These technologies have been disclosed, for example as described inJapanese Patent Laid-Open Publication No. Hei. 4-356977 and JapanesePatent Laid-Open Publication No. Hei. 5-206514.

[0006] Some of the hypotheses proposed to explain the luminescencemechanism of Si, which is an indirect transition semiconductor, are:that among the porous shapes are nanometer (nm) order three-dimensionalmicrostructure regions, which cause a loosening of the wave frequencyselection rules, causing a radiative electron-hole recombinationprocess; and that a Si polycyclic oxide (siloxane) is formed on thesurface of the porous Si, and on the interface between this siloxane andSi is a new energy level that contributes to the radiative recombinationprocess. But at any rate, it appears certain that with regard to opticalexcitation effects, a quantum confinement effect changes the energy bandstructure (broadening the gap width).

[0007] With conventional technology, however, the creation of a Simicrostructure like porous Si increases the proportion of atoms exposedon the surface, making the luminescent characteristics dependent on thesurface state. Si easily oxidizes, and oxidation of the surface changesthe band structure, changing the luminescent wavelengths and degradingthe luminescent intensity. This problem is particularly striking withporous Si, because of the instability of the hydrogen termination on thesurface.

SUMMARY OF THE INVENTION

[0008] In order to solve the above-mentioned problems of the prior art,the optoelectronic material of the present invention uses a constructionin that the surface of porous Si or Si ultrafine particles is subjectedto nitriding. This keeps the surface of the ultrafine particles frombeing oxidized in the presence of air, enabling a stable luminescence tobe obtained.

[0009] The present invention comprises a porous silicon as a firstoptoelectronic material, the surface of the porous silicon beingnitrided. This allows a stable luminescence to be obtained, withoutoxidation of the surface of the porous silicon.

[0010] The present invention also comprises silicon ultrafine particleswith particle sizes of 1-50 nm as a second optoelectronic material, thesurfaces of the silicon ultrafine particles or the entirety thereofbeing nitrided. This allows a stable electroluminescence to be obtained,without the surfaces of the silicon ultrafine particles being oxidized.

[0011] The present invention is also an optoelectronic device having anoptoelectronic material layer containing the above-mentioned first orsecond optoelectronic material, and a pair of electrodes being equippedon the top and bottom of the optoelectronic material layer. Thisconstruction provides an electroluminescence effect by injecting a smallamount of carriers by means of the pair of the electrodes in the poroussilicon or silicon ultrafine particles in the optoelectronic materiallayer and forming electron-hole pairs, creating said radiativeelectron-hole recombination process.

[0012] The present invention is also an optoelectronic conversion devicehating an optoelectronic material layer including the above-mentionedfirst or second optoelectronic material, and a pair of electrodes beingequipped on the top and bottom of the optoelectronic material layer.This provides a photodetector function by detecting changes in internalresistance or photoelectromotive force, by generating carriers by meansof light irradiation on the optoelectronic material layer.

[0013] The present invention is also a method for manufacturing anoptoelectronic material comprising the step of forming a porous siliconby anodizing a single-crystal silicon, and the step of annealing theporous silicon with an ambient gas containing at least nitrogen tonitride the surface of the porous silicon. This allows stableluminescence to be obtained, without the surface of the porous siliconbeing oxidized.

[0014] The present invention is also a method for manufacturing anoptoelectronic material comprising the step of annealing siliconultrafine particles with particle size of 1-50 nm with an ambient gascontaining at least nitrogen, at a temperature of at least 900 degreesCelsius to nitride surfaces of the silicon ultrafine particles or theentirety thereof. This allows stable luminescence to be obtained,without the surface of the silicon ultrafine particles being oxidized.

[0015] The present invention is also a method for manufacturing anoptoelectronic material comprising a target material placement step ofplacing a target material inside a reaction chamber; a substrateplacement step of placing a deposition substrate inside the reactionchamber; and an ablation step of irradiating the target material placedby means of the target material placement step with laser beam togenerate desorption and ejection of the target material; wherein thematerial in the ambient gas that has been desorped and ejected duringthe ablation stop on the target is condensed and grown, and theultrafine particles obtained therefrom are deposited on said depositionsubstrate to obtain the optoelectronic material composed of saidultrafine particles. In this method, an ambient gas containing nitrogenat a constant pressure is introduced into the reaction chamber duringsaid ablation step to nitride the surfaces or entirety of said ultrafineparticles. This construction allows the use of a high-purity targetobtained by fusion refining a single element, as well as the manufactureof an optoelectronic material with excellent stability in a single step.

[0016] Additionally, the present invention is a method for manufacturingan optoelectronic material comprising a target placement step of placinga target material inside a reaction chamber; a substrate placement stepof placing a deposition substrate inside the reaction chambers; and anablation step of irradiating the target material placed by the targetmaterial placement step with laser beam to generate desorption andejection of said target material; wherein the material in the ambientgas that has been desorped and ejected during said ablation step on thetarget is condensed and grown, and the ultrafine particles obtainedtherefrom are deposited on said deposition substrate to obtain theoptoelectronic material composed of the ultrafine particles. In thismethod, the ultrafine particles to be obtained comprise at least twodifferent elements; and a target material with the same or nearly thesame composition as said ultrafine particles is used. In the ablationstep, an inert gas is introduced into the reaction chamber at a setpressure. This construction makes it possible to manufacture anoptoelectronic material with excellent stability in a single step,without using any reactive gas.

[0017] Here, the ultrafine particles to be obtained are nitrided siliconultrafine particles, and Si_(x)N_(y) maybe used as the target.

[0018] In the above method, it is furthermore preferable to have thestep of changing the pressure at which low-pressure gas is introduced.This construction makes it possible to control the average diameter ofsaid ultrafine particles.

[0019] Thus, by employing a construction in which porous Si or Siultrafine particles are nitrided, the present invention makes itpossible to obtain stable luminescence, without oxidizing the surface ofthe luminescent Si. Furthermore, because Si₃N₄ has a larger bandgap thanthat of Si, it is possible to effectively appear quantum confinementeffects of carriers in the Si core.

[0020] Using the type of optoelectronic material mentioned above,sandwiching the optoelectronic material with a pair of electrodes, atleast one of which is in direct contact therewith, and constructingtherefrom a light emitting device or an optoelectronic conversiondevice, makes it possible to obtain an optimal electrical contactbetween the electrode and optoelectronic material layer, making itpossible to provide an effective electrolumineacence phenomenon, or aneffective photodetector function.

[0021] The optoelectronic material or optoelectronic material deviceapplication of the present invention uses a material with aninexhaustible supply and which does not cause environmental pollution,and that has excellent Si-LSI compatibility, stability, highenvironmental resistance, freedom from assembly and the like, and issuitable for a variety of multimedia-compatible devices.

[0022] Consequently, an object of the present invention is to solve theabove-mentioned conventional problems, by providing a method formanufacturing an optoelectronic material that makes it possible toobtain stable characteristics, without time decay of luminescence in theatmosphere and the like.

[0023] The following embodiments will thus be described, makingreference to the accompanying drawings, in order to further clarify theobjects and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the accompanying drawings;

[0025]FIG. 1 is a cross-sectional view showing a construction of theoptoelectronic material of a first embodiment of the present invention;

[0026]FIG. 2 shows the characteristics of the photoluminescence spectrumor the optoelectronic material of the first embodiment;

[0027]FIG. 3 shows the characteristics of the tire dependency of theintensity of the phoptoluminescence of the optoelectronic material ofthe first embodiment;

[0028] FIGS. 4(a) and 4(b) show the characteristics of the Ramanscattering spectroscopy of the optoelectronic material of the firstembodiment;

[0029] FIGS. 5(a) and 5(b) are cross-sectional views showing aconstruction of an optoelectronic material of a second embodiment of thepresent invention; and

[0030]FIG. 6 is a conceptual diagram of a manufacturing apparatus of theoptoelectronic material of the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

[0031] Below is a detailed description of the optoelectronic material,and a manufacturing method therefor, of the present invention as a firstembodiment thereof, using FIGS. 1 through 4(b).

[0032] In the present embodiment, porous Si is used as luminescent Si.Here is described the optoelectronic material formed by nitriding itssurface, and a manufacturing method thereof.

[0033]FIG. 1 shows a cross-sectional view showing a construction of theoptoelectronic material of the present embodiment. In FIG. 1, referencenumeral 11 denotes a Si single-crystal substrate; 12 a porous Si; and 13a Si nitride layer.

[0034] The manufacturing method is described using this figure. First,by anodizing p-type low-resistance (0.06-0.12Ω·cm) Si single-crystalsubstrate 11 with crystal plane orientation (100), porous Si 12 isformed on its surface. As a concrete procedure, after ultrasonic washingof Si single-crystal substrate 11 for five minutes each with acetone,methanol, and ultra-pure distilled water, the surface oxide film isremoved using a hydrofluoric acid (HF) solution diluted to 10% capacity,in order to obtain ohmic contact between the substrate and indium (In)electrodes. Then, after washing with ultra-pure distilled water forthree minutes, In backside electrodes were formed on four corners of thereverse surface. After forming the electrodes, the substrate was placedin a Teflon cell, and a coiled platinum wire was placed on the frontsurface of the substrate, forming the opposite electrode. Duringanodization, hydrogen gas is generated from the Si single-crystalsubstrate surface, which is the anode. For this reason, a localelectrical field is easily formed between the Si and solution as the Sibecomes porous, causing an uneven porous Si layer. Thus, ethanol(C₂H₅OH), which acts as a surfactant, was mixed with the HF solution, inorder to efficiently remove the hydrogen generated by the reaction tocreate porosity. When the volume ratio of HF (50% by weight) to C₂H₅OHreached 2:3, a porous material layer with a high photoluminescence (PL)efficiency and maximum porosity was obtained. After this HF solution wasstirred, it was-placed in the Teflon cell, the injection of currentthrough the solution was begun using a constant current power supply.The electric current density was about 35 mA/cm², and the current wasapplied for 10 minutes. During this process, the components wereirradiated using a 50W halogen lamp. After the process was completed,the components were left in this state for 10 minutes, and then etchingwas carried out. Next, the substrate was removed from the Teflon cell,and washed with flowing ultra-pure distilled water for three minutes.Subsequently, the In electrodes were removed from the rear surface ofthe substrate using hydrochloric acid aqueous solution diluted to 20%.Finally, the substrate was washed with ultra-pure flowing distilledwater for 3 minutes, obtaining porous Si 12.

[0035] Next, the porous Si 12 is annealed, hydrogen terminated at the Sidangling-bond is removed and the surface nitrided, forming Si nitridelayer 13. A rapid thermal annealing apparatus was used so that there waslittle change in the size of the microcrystals. Specifically, the Sisingle-crystal substrate 11 with formed porous Si 12 was placed insidethe apparatus, and after evacuating the interior of the apparatus to ahigh vacuum by bringing it to 5×10⁻³Pa using a turbo molecular pump,high-purity (6N) nitrogen (N₂) gas was introduced at 1.01/min, thenannealing was carried out for one minute at 1,100 degrees Celsius.

[0036] The optoelectronic material obtained by the above-mentionedmethod was measured using infrared absorption, XPS, PL, and Ramanscattering, to evaluate changes in structure and optical propertiesbefore and after annealing.

[0037] The results of the infrared absorption analysis of the porous Sibefore and after annealing indicated that before annealing, in peakscaused by Si-O bonds were observed, as well as peaks caused by Si-O-H,Si-H, and Si-H₂ bonds. In contrast, after annealing the peaks to whichhydrogen contributed disappeared, and only Si-O bond peaks wereobserved. These results indicate that annealing eliminated dangling-bondterminal hydrogen present on the surface of the Si microcrystals.Furthermore, XPS analysis detected N in the porous Si after annealing,seeming to indicate that the surface had been nitrided.

[0038]FIG. 2 shows the PL spectrum of the optoelectronic material of thepresent embodiment. A helium cadmium (HeCd) laser (wavelength: 325 nm;output: 15 mW) was used as the excitation light source, and measurementwas conducted at room temperature. While the peak location of the porousSi before annealing was in the vicinity of 1.9 eV, after one minute ofannealing the peak blue-shifted, to a peak in the vicinity of 2.3 eV. Inaddition, it was evident that the spectrum was spreading into thehigh-energy side. These results indicate that nitriding the surface ofthe porous Si made it possible to control the electroluminescentwavelength.

[0039]FIG. 3 shows the dependence of the PL peak strength of the presentembodiment on irradiation time. Before annealing, the intensity of theelectroluminescence of the porous Si degraded with time of irradiation.This is because the surface of the porous Si is oxidized in the presenceof air. In contrast, when annealed for one minute, theelectroluminescence was stabilized, with almost no degradation ofelectroluminescence intensity after 1-hour irradiation.

[0040] Thus, the fact that visible-spectrum PL was obtained in theoptoelectronic material of the present embodiment at room temperatureindicates that the nitride layer coating, with a bandgap that is largerthan Si, was able to express a quantum confinement effect in theelectroluminescent Si. Additionally, because the impurity diffusioncoefficient of the Si nitride layer was lower than that of the Si oxidelayer, it is believed that the surface state was stabilized by nitridingthe surface of the porous Si, enabling stable electroluminescence to beobtained, without the effects of oxide diffusion and the like, andwithout deterioration over time.

[0041] FIGS. 4 (a) and (b) show the Raman scattering spectrum of theoptoelectronic material of the present embodiment before and afterannealing. An Ar ion laser (wavelength: 514.5 nm; output: 27 mW) wasused as the excitation light source, and measurement conducted at roomtemperature. In the figure, the solid line is the result of themeasurement, and the dotted lines are the results of peak separation by2 Gaussian distributions on each spectrum. Of the two peaks, the peakshifting to the high wave frequency side was the same as bulk Si, with apeak location of 521 cm⁻¹ and full-width half maximum (FWHM) of 3-4cm⁻¹. From this, it is thought that this spectrum is due to thesubmerged Si substrate. Comparing the peaks shifting to the low wavefrequency side reveals that after one minute of annealing, the peakwidth had spread out widely. On the Raman scattering spectrum, the peaklocation shifted to the low wave frequency side due to quantumconfinement, and the peak width i n particular shifted to the low wavefrequency side. Consequently, these results indicate that the quantumconfinement effect was made more striking by nitriding the surface ofthe porous Si.

[0042] Thus, the present invention was able to obtain optoelectronicmaterial with no oxidation of the surface of the porous Si, and nodegradation of the intensity of the electroluminescence. Furthermore, itwas confirmed that annealing effectively caused a quantum confinementeffect to manifest, also enabling the control of the electroluminescencewavelength.

[0043] Note that although the present embodiment used porous Si as theelectroluminescent Si, silicon ultrafine particles with particlediameters on the nanometer order may be used.

Second Embodiment

[0044] Below is a detailed description of another optoelectronicmaterial, and a manufacturing method therefor, of the present inventionas a second embodiment thereof, using FIGS. 5(a), 5(b) and 6.

[0045] In the present embodiment are described an optoelectronicmaterial comprising ultrafine particles, the surfaces or entirety ofwhich are nitrided, and a manufacturing method therefor.

[0046] FIGS. 5(a) and 5(b) show cross-sectional views showing aconstruction of the optoelectronic material of the present embodiment.In FIG. 5(a), reference numeral 51 denotes Si ultrafine particles, and52 a Si nitride layer formed on the surface thereof FIG. 5(b) isnitrided Si ultrafine particles 53, the entirety of which is nitrided.

[0047] Next is described the method for manufacturing the optoelectronicmaterial of the present embodiment. In the present embodiment, when thesilicon ultrafine particles are deposited on the substrate, adhesiondeposition is carried out on the substrate using laser ablation of theSi in an atmosphere of gas containing nitrogen (e.g. N₂, NH₃). Note thatlaser ablation means irradiation of the target material with laser lightbeam having high energy density (pulse energy of about 1.0 J/cm² ormore), causing melting and desorption in the surface of the irradiatedtarget material, and features a nonthermal equilibrium process. Aspecific effect of nonthermal equilibrium is that it enables spatial andtemporal selection excitation. In particular, having spatial selectionexcitation characteristics allows only the material source to beexcited, while with conventional heat or plasma processing aconsiderable area or the entirety or the reaction tank was exposed toheat or ions. This makes the process clean, controlling thecontamination of impurities. Furthermore, the pulse laser excitationprocess has remarkable lower damage characteristics than the ionexcitation process with the same nonthermal equilibrium characteristics.Material desorped during laser ablation is mainly ions and neutralparticles that are atoms, molecules, and clusters (consisting of severalto several tens of atoms). The kinetic energy of this material reachesseveral tens to several hundreds of eV (electron volts) in the case ofions, and several eV in the case of neutral particles. This energy issignificantly higher than that of heat-vaporized atoms, butsignificantly lower than the energy of an ordinary ion beam.

[0048] This clean, low-damage laser ablation process is suited to thefabrication of ultrafine particles with controlled impuritycontamination, composition, crystal properties, and the like. This isbecause with the fabrication of ultrafine particles whose proportion ofsurface area is enormously large and influenced by structure, it isdispensable to provide low-damage characteristics, and when growingultrafine particles by a thermal equilibrium process, it is impossibleto prevent a wide distribution of such structural parameters as particlediameter.

[0049] Specifically, FIG. 6 is a conceptual construction diagram of theoptoelectronic material manufacturing apparatus for forming ultrafineparticles with sizes on the nanometer order, by laser ablation of the Sitarget. In FIG. 6, reference numeral 101 denotes a reaction chamber inwhich the target is placed; 102 an ultra-vacuum gas evacuation systemthat evacuates the air from inside the reaction chamber 101 to create anultra vacuum; 103 a mass flow controller that controls the flow level ofatmospheric gas supplied to reaction chamber 101; 104 a gas introductionling for supplying atmospheric gas to the reaction chamber 101; 105 agas evacuation system that evacuates atmospheric gas from inside thereaction chamber 101; 106 a target holder that holds the target; 107 thetarget; 108 pulse laser light source that irradiates laser light as anenergy beam; 109 a deposition substrate upon which material desorped andejected from the target 107 that has been excited by laser beamirradiation is deposited; 110 a laser introduction window installed onthe laser light introduction portion of the reaction chamber 101; 111 aalit to shape the laser light beam irradiated from the pulse laser lightsource; 112 a lens to condense laser light beam; and 113 a reflector todirect the irradiated laser light toward the target 107.

[0050] The operation of the optoelectronic material manufacturingapparatus having this construction is described below. In FIG. 6, first,after the ultra-high vacuum gas evacuation system 102 consisting chieflyof a turbo molecular pump creates an ultimate vacuum of about 1.0×10⁻⁹Torr in the all-metal reaction chamber 101, N₂ gas or helium (He)diluted N₂ gas (1%) is introduced therein by gas introduction line 104,via mass flow controller 103. Here, by linking operation with the gasevacuation system 105 having as its major component a dry rotary pump orhigh-pressure turbo molecular pump, the pressure of the inert gas insidethe reaction chamber 101 is set to a single pressure value in the rangeof about 0.1-50 Torr in the case of N₂ gas.

[0051] Then in this state, the surface of the Si single-crystal target107, with purity 4N, placed on the target holder 106 having a rotatingmechanism, is irradiated with laser light beam from the pulse laserlight source 108. Here, an argon fluorine (ArF) excimer laser(wavelength: 193 nm; pulse width: 12 ns: energy density: 1 J/cm²; cyclefrequency; 10 Hz) was used. At this point, a laser ablation phenomenonis generated on the surface of the Si target 107, and Si ions or neutralparticles (atoms, molecules, clusters) are desorped, and at this time,material maintaining sizes of molecules or clusters is ejected mainly inthe target normal direction, with a kinetic energy of 50 eV in the caseof ions, and 5 eV in the case of neutral particles. Next, the ejectedflying material scatters as it collides with the atmospheric gas atoms,and the kinetic energy is dissipated into the atmospheric gas, promotingassociation and agglomeration inside the chamber. Furthermore, at thesame time chemical reactions occur With the atmospheric N₂ gas in thegas phase. As a result the material is deposited on the facingdeposition substrate 109 located about 3 cm away, as nitrided Siultrafine particles ranging in size from several to several tens ofnanometers. The substrate and target temperatures are not activelycontrolled.

[0052] Note that here, N₂ gas is used as the atmospheric gas, but it isalso permissible to use another nitrogen including gas, such as NH₃. Inthis case, in order to obtain ultrafine particles with the same particlesize, it is sufficient to set the gas pressure so that the atmosphericgas has the same average gas density. For example, if NH₃ (gas density:0.75 g/l) is used as the atmospheric gas, then using N₂ (gas density:1.23 g/l) as a reference, it is sufficient to set the gas pressureapproximately 1.6-fold. Alternatively, if He diluted N₂ gas (1%)(average gas density; 0.19 g/l) is used, it is sufficient to set the gaspressure about 6.5-fold.

[0053] The structure of the deposited ultrafine particles was assessed.This showed that in the case that the deposition was conducted with 100%N₂ or NH₃ gas as the atmospheric gas, as illustrated in FIG. 5 (b),nitrided Si ultrafine particles were formed that were nitrided nearly intheir entirety. In contrast, in the case that the deposition wasconducted in He diluted N_(z) gas (1%) as the atmospheric gas, asillustrated in FIG. 5 (a), Si ultrafine particles nitrided only on thesurface were formed.

[0054] These results indicate that with the fabrication of ultrafineparticles by means of the method for manufacturing the optoelectronicmaterial of the present embodiment, it was possible to deposit nitridedSi ultrafine particles by controlling the atmospheric gas pressurethereof. Additionally, the thickness of the nitride layer can becontrolled by adjusting the composition and pressure of the atmosphericgas. In other words, the surface state can be controlled during thefabrication of ultrafine particles, by optimizing the interaction(collision, scattering, and confinement effects) between the atmosphericgas and the material (mainly atoms, ions, and clusters) ejected from thetarget by means of laser irradiation. Consequently, if the presentmethod is used, since a low-purity chemical compound formed by powdersintering or alloy target is not used, it is possible to manufactureultrafine particles using a high-purity target by fusion refining asingle element.

[0055] Furthermore, immediately after deposition, ultrafine particleshave such problems as crystal defects and the presence of unpairedelectron bonds. In such cases, in order to improve the film qualities,including crystallinity and purity, it is effective to anneal thedeposition ultrafine particles in nitrogen atmosphere at between around600 and 900 degrees Celsius.

Third Embodiment

[0056] Below is a detailed description of another method formanufacturing the optoelectronic material of the present invention as athird embodiment thereof.

[0057] In the present embodiment is described a method for manufacturingan optoelectronic material comprising ultrafine particles nitrided intheir entirety. As with the second embodiment, an excimer laser is usedas the light source, and using the optoelectronic material manufacturingapparatus shown in FIG. 6, laser ablation is carried out on the Si₃N₄target, forming Si₃N₄, ultrafine particles.

[0058] Specifically, in FIG. 6, first, after the ultra-high vacuum gasevacuation system 102 consisting mainly of a turbo molecular pumpcreates an ultimate vacuum of 1.0×10⁻⁹ Torr in the all-metal reactionchamber 101, ultra-pure (6N) helium (He) is introduced therein by thegas introduction line 104, via the mass flow controller 103. Here, bylinking operation with the gas evacuation system 105 having as its majorcomponent a dry rotary pump or high-pressure turbo molecular pump, thegas pressure inside the reaction chamber 101 is set to a single pressurevalue in the range of about 0.1-100 Torr.

[0059] Then in this state, the surface of the Si₃N₄ powder sinteredtarget 107, with purity 4N, placed on the target holder 106 having arotating mechanism, is irradiated with laser light beam from the pulselaser light source 108. Here, an ArF excimer laser (wavelength: 193 in;pulse width; 12 ns: energy density: 1 J/cm²; cycle frequency: 10 Hz) wasused. At this point, a laser ablation phenomenon is generated on thesurface of the Si₃N₄ target 107, and ions or neutral particles (atoms,molecules, clusters) are desorped, and at this time, materialmaintaining sizes of molecules or clusters is ejected mainly in thetarget normal direction, with a kinetic energy of 50 eV in the case ofions, and 5 eV in the case of neutral particles. Next, the ejectedflying material scatters as it collides with the atmospheric gas atoms,and the kinetic energy is dissipated into the atmospheric gas, promotingassociation and agglomeration inside the chamber. As a result, thematerial is deposited on the facing deposition substrate 109 locatedabout 3 cm away, as Si₃N₄ ultrafine particles. The substrate and targettemperatures are not actively controlled.

[0060] Note that here, He gas is used as the atmospheric gas, butanother inert gas, such as Ar may be used. In this case, it issufficient to set the gas pressure to the same average gas density. Forexample, if Ar (gas density: 1.78 g/l) is used as the atmospheric gas,then using Hie (gas density; 0.18 g/l) as a reference, it is sufficientto set the gas pressure approximately 0.1-fold.

[0061] The structure of the deposited ultrafine particles was assessed.As illustrated in FIG. 5 (b), Si₃N₄ ultrafine particles were formed thatwere nitrided nearly in their entirety.

[0062] These results indicate that with the fabrication of ultrafineparticles by means of the method for manufacturing the optoelectronicmaterial of the present embodiment, it was possible to deposit Si₂N₄ultrafine particles with nearly the same composition as the target, bycontrolling the atmospheric gas pressure thereof, even when using aninert gas that does not contain nitrogen. In other words, by optimizingthe interaction (collision, scattering, and confinement effects) betweenthe inert gas and the material (mainly atoms, ions, and clusters)ejected from the target through laser irradiation, it is possible toform crystal compound ultrafine particles.

[0063] Here some observations on the effects of the atmospheric gas onlaser ablation will be made. The material ejected from the targetsurface by means of laser irradiation maintains the composition of thetarget without being vaporized, and is propagated maintaining a straightline, mainly in the form of atoms and ions. In the presence ofatmospheric gas, however, collisions cause scattering and rob energyfrom the material, changing the spatial dispersion at deposition,deposition speed, the distribution of kinetic energy of the depositionmaterial, and the like. These changes differ depending on the type andkinetic energy of the ejected material. In general, it is believed thatbecause heavier material (here, Si) is loss susceptible to scattering,it maintains a straight path even during laser ablation. As a result, ifdeposition is carried out under low gas pressure, the material reachesthe substrate in a state lacking nitrogen, which is susceptible toscattering and also has high vapor pressure.

[0064] At first, the atoms and ions ejected from the target travel atdifferent speeds, but as the atmospheric gas pressure rises, they aremore likely to collide with the atmospheric gas and scatter, causingtheir speed to slow, at the same time approaching a uniform speed. As aresult, the ejected material is confined within a given space,controlling the lack of nitrogen that was occurring at low gaspressures. Because during laser ablation in an inert gas atmosphere, theonly nitrogen supplied to the deposition material is that which has beenejected from the target, this effect is vital.

[0065] At the sane time, when laser ablation is conducted in a highpressure gas atmosphere, the atmospheric gas is compressed, and itspressure and temperature raised, forming a shock front. Hence, someobservations on the effects of this shock front on nitride formationwill be made. Nitrided Si is formed in accordance with the followingformula.

3Si+2N₂→Si₃N₄   (Formula 1)

[0066] The increase in gas pressure promotes the formation of Si₃N₄ (thereaction progressing to the right in Formula 1) , which is a reactionthat brings about a reduction in mass and molar number. The increase intemperature thermally promotes the excitation of the ejected material.The increase in temperature, however, also works in the direction ofincrease of the generation energy of Si₃N₄, inhibiting the formationthereof. As the shock front proceeds forward and its distance from thetarget increased, the pressure and temperature decline.

[0067] Additionally, the generation energy decreases as the temperaturefalls. As a result of the above, a region meeting sufficiently lowgeneration energy conditions and at the same time having a hightemperature state is formed a certain distance from the target, andnitride reactions are promoted within this region. In other words, it isbelieved that the crystal cores of Si₃N₄ maintaining stoichiometry areformed in the region that promotes this gas-phase nitriding. Then withfurther airborne motion, the material rapidly cools as it agglomerates,reaching the substrate and providing Si₃N₄ ultrafine particles.

[0068] If the deposition substrate is placed so that it is in contactwith this nitriding promotion region, the substrate surface becomes anactive region, and migration of the crystal core generated by gas phaseon the substrate is thought to cause orientation and crystallization.Conversely, if the deposition substrate is placed so that it is outsidethis nitriding promotion region, the microcrystals grown in gas phasereach the substrate while associating, which results in a non-orientedstructure.

[0069] As is clear by the above observations, with laser ablation thereis an interrelation between atmospheric gas pressure (P) and thedistance between target and substrate (D). The material ejected from thetarget by means of laser irradiation goes into a plasma slate called aplume. Because this plume is affected by collisions with the atmosphericgas, the size of the plume is dependent on the gas pressure: the higherthe gas pressure, the smaller the plume. Furthermore, the features ofthe substrate deposition material depends greatly on the speed of thematerial ejected from the target when it reaches the depositionsubstrate. For this reason, in order to obtain the same characteristics,the value PD_(n) must be in a constant relationship as a processcondition for keeping the above-mentioned speed constant. Here, thevalue n is assumed to be between about 2 and 3. Consequently, forexample in the case that D is doubled, the corresponding gas pressuremay be set to about ¼-⅛.

[0070] Thus, in the optoelectronic material manufacturing method of thepresent embodiment, if laser ablation is conducted using a targetmaterial consisting of a material including an element with high vaporpressure (here, nitrogen), then in order to prevent stoichiometry frombeing altered when the element with high vapor pressure is removed, amethod of supplementing the atmospheric gas with a high vapor-pressureelement using a gas including a high vapor-pressure element is not used.Rather, a plume of the appropriate size is formed, by adjusting theatmospheric gas pressure and the distance between the target anddeposition substrate, and forming ultrafine particles which maintainstoichiometry. In other words, inside a plume of the right size, theloss or elements with high vapor pressure is prevented, formingultrafine particles on the deposition substrate with nearly the samecomposition as the target. Consequently, with the optoelectronicmaterial manufacturing method of the present embodiment, the atmosphericgas pressure and distance between the target and deposition substrateare freely set to ensure that the plume with the appropriate size isformed.

[0071] When this method is used, it is possible to adjust the pressureof the atmospheric gas, or in other words to adjust the number ofcollisions between the material desorped from the target material andthe atmospheric gas atoms, and control the proportion of element withhigh vapor pressure formed inside the plume and confined inside thehigh-temperature, high-pressure region, thereby controlling thecharacteristics of the substrate deposition material.

[0072] Furthermore, immediately after deposition, ultrafine particleshave such problems as crystal defects and the presence of unpairedelectron bonds. In such cases, in order to improve the film qualities,including crystallinity and purity, it is effective to anneal thedeposition ultrafine particles in nitrogen atmosphere at between around600 and 900 degrees Celsius.

[0073] Note that in the description above, a method for manufacturingSi₃N₄ ultrafine particles, which are two-element nitride ultrafineparticles, were described. It is also possible, however, to use suchsubstances as oxides as the target material for fabricating theultrafine particles. Needless to say, it is also possible to usecompounds consisting of three or more elements.

[0074] The present invention has been described, based on the preferredembodiments shown by the drawings. To a person skilled in the art,however, it would clearly be obvious to modify and/or change the presentinvention, and such modifications are included in the scope of thepresent invention.

What is claimed is:
 1. An optoelectronic material comprising a poroussilicon, the surface of said porous silicon being nitrided.
 2. Anoptoelectronic material comprising silicon ultrafine particles havingparticle diameters of 1-50 nm, the surfaces of said silicon ultrafineparticles or the entirety thereof being nitrided.
 3. A light emittingdevice having: an optoelectronic material layer including anoptoelectronic material comprising a porous silicon, the surface of saidporous silicon being nitrided; and a pair of electrodes equipped on thetop and bottom of said optoelectronic material layer.
 4. A lightemitting device having: an optoelectronic material layer including anoptoelectronic material comprising silicon ultrafine particles havingparticle diameters of 1-50 nm, the surfaces of said silicon ultrafineparticles or the entirety thereof being nitrided; and a pair ofelectrodes equipped on the top and bottom of said optoelectronicmaterial layer.
 5. An optoelectronic conversion device having: anoptoelectronic material layer including an optoelectronic materialcomprising a porous silicon, the surface of said porous silicon beingnitrided; and a pair of electrodes equipped on the top and bottom ofsaid optoelectronic material layer, said device having a photodetectorfunction by detecting a change in internal resistance orphotoelectromotive force due to the generation of carriers via lightirradiation on said optoelectronic material layer.
 6. An optoelectronicconversion device having: an optoelectronic material layer including anoptoelectronic material comprising silicon ultrafine particles havingparticle diameters of 1-50 nm, the surfaces of said silicon ultrafineparticles or the entirety thereof being nitrided; and a pair ofelectrodes equipped on the top and bottom of said optoelectronicmaterial layer, said device having a photodetector function by detectinga change in internal resistance or photoelectromotive force due to thegeneration of carriers via light irradiation on said optoelectronicmaterial layer.
 7. A method for manufacturing an optoelectronic materialcomprising the steps of; forming a porous silicon by means of anodizinga single-crystal silicon; and annealing the porous silicon in an ambientgas including at least nitrogen to nitride the surface of said poroussilicon.
 8. A method for manufacturing an optoelectronic materialcomprising the step of annealing silicon ultrafine particles havingparticle diameters of 1-50 nm, in an ambient gas containing at leastnitrogen, at a temperature of at least 900 degrees Celsius to nitridesurfaces of said silicon ultrafine particles or the entirety thereof. 9.A method for manufacturing an optoelectronic material comprising: atarget material placement step of placing a target material inside areaction chamber; a substrate placement step of placing a depositionsubstrate inside the reaction chamber; and an ablation step ofirradiating the target material placed during said target materialplacement step with laser light beam, to generate desorption andejection of said target material; thereby the material desorped andejected during said ablation step on the target being condensed andgrown in the ambient gas, and the ultrafine particles obtained therebybeing deposited on said deposition substrate to obtain theoptoelectronic material composed of said ultrafine particles, wherein anambient gas is introduced into the reaction chamber at a constantpressure during said ablation step to nitride the surfaces of saidultrafine particles or the entirety thereof.
 10. A method formanufacturing an optoelectronic material comprising: a target materialplacement step of placing a target material inside a reaction chamber; asubstrate placement step of placing a deposition substrate inside thereaction chamber; and an ablation step of irradiating the targetmaterial placed during said target material placement step with laserlight beam to generate desorption and ejection of said target material;thereby, the material desorped and ejected during said ablation step onthe target being condensed and grown in the ambient gas, and theultrafine particles obtained thereby being deposited on said depositionsubstrate to obtain the optoelectronic material composed of saidultrafine particles, wherein the ultrafine particles to be obtained arecomposed of at least two elements, and using the target material withthe same or nearly the same composition as said ultrafine particles,during said ablation step, inert gas is introduced into the reactionchamber at a constant pressure.
 11. The method for manufacturing anoptoelectronic material according to claim 10, wherein the ultrafineparticles to be obtained are nitrided silicon ultrafine particles, andSi_(x)N_(y) is used as the target.
 12. The method for manufacturing anoptoelectronic material according to any of claims 9 through 11, furthercomprising a step of changing the pressure at which low-pressure gas isintroduced to control the average particle diameter of said ultrafineparticles.
 13. The optoelectronic material manufactured by the methodfor manufacturing an optoelectronic material according to any of claims7 through 12.